1. Technical Field
The disclosure relates to a magnetic random access memory. Particularly, the disclosure relates to a spin transfer torque magnetic random access memory having a perpendicular magnetization.
2. Technical Art
A basic structure of a magnetic random access memory (MRAM) is formed by a pinned layer, a barrier layer and a free layer. By changing a magnetization direction of the free layer to be parallel or antiparallel to a magnetization direction of the pinned layer, a magnetoresistance thereof respectively have a low resistance state and a high resistance state for storing information.
A write operation of the conventional MRAM (for example, a mass-produced Standalone MRAM) can be implemented through a magnetic field induced by a current conducted by wires. Regarding a field-switching MRAM, the magnetic field generated by a write wire thereof is liable to interfere adjacent bits to cause write error, and as a device size decreases, a required switching field is increased, which is of no avail for miniaturization. Therefore, the MRAM at a technology below 65 nm node applies a spin transfer torque (STT) mechanism, by which an angular momentum conservation mechanism of spin-polarized electrons and a local magnetic moment is used to switch the magnetization direction of the free layer of the device, so as to implement the write operation. A STT write current is proportional to a device size, which avails miniaturization, though when the device size is reduced, magnetic anisotropy energy (KuV) stored in the free layer is also reduced, which is liable to be interfered by a random thermal perturbation field generated by temperature, so that thermal stability of the device is influenced. Although the thermal stability of the device can be improved by selecting a high saturation magnetization material or increasing a thickness of the free layer, the current required for switching is also increased. Therefore, it is an important issue in development of STT-MRAM to reduce the switching current while simultaneously maintaining a suitable thermal stability.
The MRAMs can be divided into in-plane magnetization anisotropy (IMA) MRAMs and perpendicular magnetization anisotropy (PMA) MRAMs according to material characteristics thereof. FIG. 1 is a structural cross-sectional view of a conventional IMA MRAM. Referring to FIG. 1, the IMA MRAM structure includes a magnetic pinned layer 100 having a fixed magnetization direction 102, which is not varied along with an external magnetic field, and the fixed magnetization direction 102 is used as a reference. A tunneling insulation layer 104 is disposed on the magnetic pinned layer 100. A magnetic free layer 106 is disposed on the tunneling insulation layer 104. The magnetic free layer 106 has a magnetization direction 108 capable of being switched. The magnetization direction 108 of the magnetic free layer 106 can be freely changed to be parallel or antiparallel to the magnetization direction 102 by applying the external magnetic field or current. By measuring a magnetoresistance difference generated due to parallel and antiparallel of the magnetization directions between the magnetic free layer 106 and the magnetic pinned layer 100, bit data stored in the magnetic free layer 106 can be determined.
FIGS. 2A-2B are cross-sectional views of PMA MRAM structures. Referring to FIG. 2A, the PMA MRAM structure is basically formed by staking a magnetic pinned layer 110, a tunneling insulation layer 112 and a magnetic free layer 114. A magnetization direction 120 of the magnetic pinned layer 110 is a fixed direction perpendicular to its horizontal plane. A magnetization direction 122 of the magnetic free layer 114 can also be freely switched between two directions by applying the external magnetic field or current, though it is perpendicular to its horizontal plane.
However, regarding the IMA MRAM structure of FIG. 1, if a perpendicular magnetic material is used to directly replace an in-plane magnetic material to form the PMA magnetic magnetoresistance device of FIG. 2, since an ordering direction of a multilayer film of many perpendicular magnetic materials such as Co/Pt, Co/Ne, Co/Pd, etc. is face-centered cubic (f.c.c) (111), when MgO is used as a material of the tunneling insulation layer 112, a b.c.c (002) ordering direction required for a high magnetoresistance (MR) ratio cannot be formed at the MgO interface. A low MR ratio may limit an operation speed of the device, so that a suitable polarization enhancement insertion layer is added between the perpendicular magnetic material layer and the MgO tunneling insulation layer 112 to form the ordering direction required for the high MR ratio at the MgO interface, so as to improve the MR ratio of the PMA magnetic magnetoresistance device. Referring to another PMA MRAM structure of FIG. 2B, based on the structure of FIG. 2A, insertion layers 116 and 118 are added between the perpendicular magnetic material layer and the MgO tunneling insulation layer 112. The insertion layers 116 and 118 generally have a material of CoFeB, etc., which has a high polarization and can form (002) ordering direction at the MgO interface. Although these materials are belonged to the in-plane magnetic material, based on a strong exchange coupling there between, the perpendicular magnetic material may force the magnetic moment of the in-plane magnetic material insertion layer to erect as the perpendicular magnetization direction.
Anisotropy energy of the perpendicular magnetic material is large, so that when the device size is highly miniaturized, the thermal stability thereof is still maintained. For example, regarding the perpendicular magnetic material used by a hard disc, a size of a grain thereof is only a dozen nm, though a thermal stability coefficient (KuV/kBT) thereof is still greater than 60. Therefore, to use the PMA MRAM having the perpendicular magnetization direction is regarded as a key technique with a great potential for further miniaturization. However, since a damping constant of the perpendicular magnetic material is generally greater than that of the in-plane magnetic material, when the free layer of the perpendicular magnetic material is switched by the STT, a critical current density (Jc) required for the switching operation is greater than that of the in-plane magnetic material. If the critical current density required for switching the free layer of the perpendicular magnetic material can be further reduced, it avails a mass production of the PMA MRAM.